Field of the Invention
The field of the invention relates to a catalyst especially for a fuel cell and autocatalyst and electrode surface coating and electroplating processes for deposition of surface coating, especially in a fuel cell and a catalyst.
Description of Related Art
Platinum has often been used as a preferred catalyst material for electrodes in fuel cells, especially fuel stacks for cars, and in autocatalysts. A catalyst, especially a platinum catalyst is a crucial part in a fuel cell. The catalytic reaction takes place on the surface of the electrodes. The electricity is created by the catalytic reaction when a fuel such as Hydrogen is electrochemically oxidized to protons on the surface of the anode. Platinum is known to be an excellent catalyst for fuel cells; however it is a very expensive material.
Since platinum has a smooth surface and its surface area is limited by the geometry of the electrode, it is not efficient for transferring electrical charge. The platinum with a smooth surface is hereinafter called shiny platinum.
An electrode which is intended for long term use with a nonrenewable energy source must require minimal energy—a high electrode capacitance and correspondingly low electrical impedance is of great importance.
It is known that a catalyst applied on an electrode surface accelerates the electrode reactions and that the transfer current is proportional to the surface area of the electrode. Many attempts are reported trying to improve the ability of a catalyst converting fuel to electricity. Those attempts try to increase the surface area of the electrode without increasing the amount of the expensive platinum catalyst material.
One approach to increase the surface area of a platinum electrode without increasing the electrode size is to electroplate platinum rapidly such that the platinum molecules do not have time to arrange into a smooth, shiny surface. The rapid electroplating forms a platinum surface which is commonly known as platinum black. Platinum black has a porous and rough surface which is less dense and less reflective than shiny platinum. U.S. Pat. No. 4,240,878 to Carter describes a method of plating platinum black on tantalum.
Platinum black is more porous and less dense than shiny platinum. Platinum black has weak structural and physical strength and is therefore not suitable for applications where the electrode is subject to even minimal physical stresses. Platinum black also requires additives such as lead to promote rapid plating. Finally, due to platinum black's weak structure, the plating thickness is quite limited. Thick layers of platinum black simply fall apart.
Fuel stacks for cars use about 2 oz of platinum group metals per unit. Pure platinum catalysts are used for hydrogen fueled fuel cells, while alloys of platinum with ruthenium are typically used for reformed hydrocarbon fuel cells to improve the tolerance of the catalyst to carbon monoxide.
The fuel cell research estimates that loadings can be reduced to about 1 oz per unit through better utilization of platinum and thinner deposition layer. Other estimates show that when fuel cells are commercially produced each engine will require between 0.2 and 0.3 oz platinum per unit.
The main consumer of world platinum group metal supply is the automobile industry. 41% of platinum demand in 2001 was accounted for autocatalyst use. Platinum group metals are used in autocatalysts to facilitate the removal of three of the main combustion byproducts CO, hydrocarbons, and NOx. The use of platinum is increased due to strong growth in production and sales for diesel cars. Diesel autocatalysts only use platinum rather than the mixture of platinum and palladium commonly used in gasoline catalysts.
Platinum is the most common catalyst for fuel cells. However, due to its high cost it is often doped with palladium, ruthenium, cobalt, or more recently iridium or osmium. In addition to its high cost, platinum is also quite rare. In fact, there is not enough platinum in the world to equip every vehicle in use today with a traditional platinum catalyst proton exchange membrane fuel cell. For this reason, there is a high desire to develop new catalysts, and new platinum deposition techniques to reduce the amount of platinum needed for fuel cell catalysts and autocatalysts in general.
For the foregoing reasons there is a need for an improved platinum surface coating and process for coating the surface to obtain an increased surface area for a given geometry and at the same time the coating is structurally strong enough to be used in applications where the platinum surface coating is subject to physical stresses.